The present invention relates to a method for producing bipolar transistors, and more specifically relates to transistors in which the base region and emitter region are produced by diffusion.
There is conventionally a known method of producing a bipolar transistor in which a collector region, base region and emitter region are successively superposed on a semiconductor substrate. In this method, the base region is formed by depositing a semiconductor film layer on the preceding collector region to a predetermined thickness, and then by introducing an impurity into the semiconductor film layer at a predetermined concentration. The introduction of the impurity can be effected by ion implantation or conventional diffusion technology.
With recent developments in integrated circuit technology, the size of bipolar transistor has been significantly reduced. In addition, the thickness of the base region has been made smaller and smaller in order to increase operating speed of the bipolar transistor. When introducing an impurity into a thin semiconductor film layer to form such thin base region by ion implantation, there is the drawback that a steep impurity density distribution cannot be easily obtained since the impurity has a Gaussion distribution in the depth direction. On the other hand, when the conventional diffusion technology is utilized to carry out the introduction of impurity, there is caused the drawback that a base region having desired characteristics cannot be obtained since the introduction amount is not controlled accurately.
In the above noted conventional method of producing a bipolar transistor, the emitter region is formed by depositing a semiconductor film layer on the underlying base region and then by introducing an impurity into the semiconductor film layer. Otherwise, the emitter region is formed by introducing one conductivity type of impurity into a semiconductor film layer disposed on the collector region after introducing an impurity of opposite conductivity type into the same semiconductor film layer to form the base region. The introduction of impurity is conventionally effected by ion implantation into the semiconductor film layer.
The ion implantation is carried out by firstly ionizing impurity atoms, then mass-filtering the ions, and thereafter accelerating the ions to a given energy to implant the ions into the semiconductor substrate. Ion implantation allows the impurity introduction amount to be controlled accurately, and allows an impurity to be easily doped through an insulating film into the semiconductor substrate. For this reason, ion implantation is commonly utilized for conventional impurity doping in bipolar transistor fabrication processes.
However, when using ion implantation for forming the emitter region, there is caused the drawback that such ion implantation will adversely affect the base region which is disposed under the semiconductor film layer to be ion-implanted, since the impurity ions are accelerated and irradiated onto the semiconductor film layer. Namely, impact due to the accelerated impurity ions would cause the so-called emitter push effect to deform the diffusion distribution of an impurity in the base region, to thereby degradate performance of the bipolar transistor.